Analog cellular radiotelephone systems utilize sub-audible signalling alongside the speech for supervision of the network. Thus, while any given radiotelephone user is speaking, sub-audible data messages are exchanged between his radiotelephone and the network to prepare for handoffs from cell-to-cell, for power level changes, and the like. This sub-audible signalling is also utilized to maintain continuity of the radio link between the cell site and the radiotelephone. Previous analog cellular radiotelephone systems utilized a pseudo-continuously transponded Supervisory Audio Tone (SAT), a unique sub-audible tone assigned to each cell, to differentiate the intended cell from surrounding ones and to maintain continuity of the radio link between the cell and the radiotelephone; loss of this transponded and returned tone would cause both the radiotelephone and the network to mute their audio to prevent noise from being heard by either listener. In split-channel analog cellular, these SAT tones interfered with one another and were replaced by digital SAT (DSAT)--see Levine U.S. Pat. No. 4,984,290 U.S. Ser. No. 228,071, filed Aug. 4, 1988, incorporated herein by reference thereto.
The problem is that a properly selected DSAT (with minimum hamming distance and not susceptible to falsing, according to the incorporated disclosure) is shorter than many data messages. Accordingly, interleaved, sub-audible data messages required for network supervision would first be detected as a loss of DSAT and would mistakenly cause voice muting. The challenge then is to have data messages not false the voice muting process.
This invention takes as its object to overcome these shortcomings and to realize certain advantages presented below.